Method of manufacturing low thermal-expansion porcelain

ABSTRACT

A heat resistant porcelain body having a smooth, dense and hard glaze layer on the surface thereof is prepared by coating a porcelain body comprised of at least one of petalite and spodumene in admixture with plastic clay and containing specified amounts of A12O3, SiO2 and LiO2 in addition to at least one of 0.5-6 percent by weight of ZnO and 7-26 percent by weight of free SiO2 with a glaze preparation containing primarily SiO2 and thereafter firing the coated porcelain body at 1250*-1350* C.

United States Patent Motoyuki [451 Mar. 21, 1972 [54] METHOD OF MANUFACTURING LOW 3,170,805 2/1965 McMillan et al ..106/48 x THERMAL-EXPANSION PORCELAIN ,217 l/1969 3,449,203 6/1969 [721 WWW", JaPa" 3,458,331 7/1969 Kroger ..106/45 [73] Assignee: Nippon Toki Kabushiki Kaisha, Nagoya- 3,480,452 11/ 1969 shi, Aichi-ken, Japan g/ 22 Filed: Aug. 8, 1969 I [21 1 AppL NOJ 848,693 FOREIGN PATENTS OR APPLICATIONS ReMedU S Application I). 239,249 7/1962 Australia ..117/125 63 Continuation-impart of Ser. No. 499,022, 0m. 20, Primary Examiner-William D-Marfin 1965, Pat. No. 3,499,787, {issz'stant Examiner- M. R. Lusignan A1torney-Stevens, Davis, Miller & Mosher {52] [1.8. CI ..117/118, 65/311,65/62,

106/45, 106/46, 106/48, 117/125 [57] ABSTRACT 2 A heat resistant porcelain body having a smooth, dense and I 1 a are 1 1 06/39 hard glaze layer on the surface thereof is prepared by coating a porcelain body comprised of at least one of petalite and spodumene in admixture with plastic clay and containing 1561 specified amounts of A1 0 sio and Lio in addition to at UNITED STATES PATENTS least one of 0.5-6 percent 11y weight of ZnO and 7-26 percent by weight of free 810: with a glaze preparation containing g ggpp primarily Si0 and thereafter firing the coated porcelain body i i Y at l250-1350 C. 2,919,995 [/1960 Landron ....106/65 X 3,0l9,l 16 l/ 1962 Doucetto ..106/48 X 6 Claims, No Drawings 1 METHOD OF MANUFACTURING LOW THERMAL- EXPANSION PORCELAIN This application is a continuation-iri-partof application Ser. No. 499,022, filed Oct. 20, v1965, now US. Pat. No. 3,499,787, dated March 10, 1 -970.

This invention relates to a process for manufacturing a heatresisting porcelain body having a smooth, dense and hard glaze layer on the surfacethereof, the porcelain body having a low thermal-expansion coefficient which .is more especially, in the range offrom 3.9 X l to 0.

Hitherto, a great difficulty has been experienced in attempt ing to coat the surface of a porcelain body having a low coefficient of thermal-expansion with a glaze having a still lower coefficient of thermal-expansion than that of the porcelain body. For such glazing purposes, various attempts have been made including coating with very thin glassy layers, self-glazing and cracked glazing, butnone has proved satisfactory.

The formation of thin glassy films involves difficulties in insuring uniform thickness and tends to cause cracks in thick portions of the film. Self-glazing isIquite susceptible to variations in heating or cooling conditions and the glazing process makes it difficult and impracticable to obtain products of stable qualities. Cracked glazing also has the disadvantages, in

that the tensile stress due tothe'difference in coefiicierit of thermalexpansion between the glaze and the body is exerted on the surface of the porcelain body to lower the thermal shock resistance producing cracks, which tend to collect impurities leading to sanitation problems and inferior appearance.

An object of this invention is to provide a porcelain body having a low coefficient of thermal-expansion with a glaze layer having a low coefficient of thermal-expansion.

Another object of this invention is to provide a porcelain body with a smooth, dense, hard, and crack-free glaze layer which has a low coefficient of thermal-expansion.

A further object of this invention is to provide a process for manufacturing a porcelain body which possesses the aforesaid characteristics.

According to the present invention there is provided a process for manufacturing a porcelain'body' having a low coefficient of thermal-expansion and being provided with a smooth, dense and hard glaze layer thereon, which comprises (a) coating a porcelain body with a glaze-forming material, the composition of said porcelain body andthe glaze-forming material'being such that when the coating ofthe glaze-forming material applied to said porcelain body is heated, components of said porcelain body and glaze-forming. material react with each other during the heating thereof to form, in the glaze, crystals having a low coefficient of thermal-expansion and (b) heating the coated porcelain body 'to vitrify and simultaneously to produce a quantity of saidcrystals having a low thermal-expansion coefficient in the glaze, whereby, on-cooling a smooth, dense and hard glaze layer is formed on the'porcelain body. 1

Thus, the objects of the inventionareaccomplished by (a) coating a green or biscuit body surface of porcelain with a glaze-forming material which will react with the main constituents or additives of the body to form crystals oflow thermal-expansion coefficient in the glaze, and (b) firing the coated body to vitrify and also to form a glaze layer which contains a substantial amount of crystals with a low coefficient of thermal-expansion on the surface of said porcelain body, thereby on cooling obtaining a smooth, dense, and hard porcelain which is also non-porousand possesses a good thermal shock resistance.

Specifically, in order to obtain the above desired objectives, a porcelain body is made up of a composition comprising at least one mineral selected from the group consisting of petalite and spodumene in admixture with plastic clay,

preferably kaolin. When using petalite it should be employed in an amount of about 40-90 percent by weight with the clay being present in an amount of about 10-60 percent by weight. When using spodumene this should be employed in an amount of 30-60 percent by weight with the clay being about 40-70 percent by weight. The mixture should contain 55-74 percent by weight of SiO 18-32 percent by weight of M 0 and about 1.5-4.5 percent by weight of Li 0. When spodumene and petalite are used in admixture their relative proportions can vary widely, it being only necessary that the content of Li,O derived from the petalite and spodumene comprise about 1.5-4.5 percent by weight of the body composition. These proportions, of course, are easily calculated from the known Li O content of spodumene and petalite which are listed hereinafter. Normally the composition will also contain small amounts of K 0 and Na o, e.g., l-3 percent since these later oxides are normally present in the lithium aluminum silicates known as petalite and spodumene and in plastic clay. The ignition loss of the composition will usually be between 1.5 and l 1 percent by weight, after firing. K 0 and Na O, however, are not essential constituents.

It is necessary to produce large quantities of low thermal-exp'ansion crystals in the glaze-forming material in order that the resulting glaze layer is suitable as such for the low thermal-expansion porcelain body on which it is formed. If a body contairiing Li O in a large amount, such as a petalite-clay or spodu'm'ene clay body, is coated with a glaze-forming material and then'fired, a large amount of Li+ ions will be diffused from the body to the glaze-forming material to promote the fusion thereof and consequently hinder the glaze-forming material from c'rystallizing, thereby making it impossible for the resulting glaze layer to have low thermal-expansion property. In order to solve this problem, it is necessary that the body contains free silica in amounts of 7-26 percent. If the originalbody contains less than 7 percent of free silica, additional free silica must be incorporated, as an additive, in the original body to the extent that it contains at least 7 percent of free silica, or else 0.5-6 percent of ZnO must be added to the original body. However, the use of more than 26 percent of free' silica in the body will increase the thermal-expansion coefficient of the fired body and deteriorate the heat resistance of the porcelain product so obtained. The addition of more than 6 percent of ZnO to the body is undesirable because this will cause the firing temperatures to be lowered and a deformedfir'ed body to be produced.

The constituents of the composition discussed above are products of nature, the principal composition of which is as follows:

Plastic clay [gloss l3-l 5% sio, 48-5 1% A1,0 3 143% k,oNa,o 14% Petalite SiO, 75-77% Aho; l 5-] 7% Li,o 3-4; loo-Nap 1% Kaolin 15;1055' l I-l 4% sio, 45-48% 41,0, 35-38% K,0+Na,0 14% As is seen from the above, there is some variation in the amount of the individual compounds of each of the minerals petaliteand spodumene and clay. Therefore, it will be seen that the specific amount of petalite or spodumene within the percentages recited above which are listed with the plastic clay within the recited percentages must be such that the amounts of the critical constituents SiO- A1 0; and U 0 fall within the ranges set forth above when the composition is prepared. The plastic clay such as kaolin also contributes quantities of SiO- 'and A1 0 which amounts are to be included within the percentages set forth. The amount of the mixture of the minerals (petalite and spodumene) and the clay in the porcelain body should constitute about 74-995 percent by weight of the body.

An essential feature of the present invention is that there be included in the porcelain body certain additives, namely zinc oxide in an amount of 0.5-6 percent, preferably 2-5 percent, based on the total weight of the porcelain body and/or free SiO in an amount of 5-20 percent, preferably 7-15 percent by weight based on the total weight of the porcelain body. It is to be noted that the additive-incorporated porcelain body contains free SiO in amounts of 7-26 percent by weight because the original body contains SiO in amounts of 2-6 percent by weight.

By including these additives in the indicated amounts, the objectives of the present invention are achieved by virtue of reaction between the components of the body and the components of the glaze composition which will be hereinafter described in order to form a substantial amount of crystals within the glaze having a low thermal coefficient of expansion and composed principally of Silica solid solution crystals whose end members are composed of ,B-quartz and ,8- eucryptite, or B-spodumene solid solution crystals.

Though the porcelain body coated according to the process of this invention has been defined in terms of its essential ingredients, namely the mineral, the plastic clay, e.g., kaolin, and the additives ZnO and/or free SiO it must be recognized that minor amounts of conventional additives for the promotion of vitrification or for the extension of the firing temperature range may also be present, for example, the oxides, carbonates or silicates of barium, magnesium, calcium, strontium and lead as well as feldspar, nepheline and talc. Normally these additives will make up less than percent of the total porcelain body. The actual formation of the porcelain body is conducted in a conventional manner, utilizing the well known jiggering or casting methods.

According to the next step of the instant process, the surface of the formed green body or its biscuit is coated with a glaze preparation comprising 60-79 percent SiO 9-12 percent A1 0 0-20 percent ZnO; 0-7 percent MgO; 2-5 percent Cao: 0-7 percent Li O (all molar percentages) and traces of K 0 and Na O and other compounds. The application of the glaze to the green body or biscuit is also a conventional step involving preparation of an aqueous slurry of the finely divided glaze constituents and application thereof to the surface of the green or biscuit body. The coated body is then fired at 1250-l350 C. whereby the coated body is vitrified and simultaneously the glaze layer is formed containing a large amount of Silica O or ,B-spodumene solid solution crystals. Various oxides of the glaze preparation listed above are conveniently added in the form of natural existing minerals or clays. Thus, for example, the glaze composition may include silica sand, calcined kaolin, alkaline feldspar, alumina, petalite, talc, dolomite, limestone and zinc oxide. These materials which may be used in the preparation of the glaze are composed principally as follows:

Silica Sand SiO: 94-97% A1 0, l-27r loo-Nap l-27l Alkaline feldspar SiO: 155-6754 M 0: l6-l87r K,O-Na,O l2-l47t Pelalitc sio 75-77% A1 0 -17% Li O 3-47: K,O'Na,O l7r Kaolin (calcined) SiO, 54-56% AI O 38-427: K,O+Na O 2-35 Alumina A1 0, 99%

Talc (calcined) SiO 65-672 MgO 32-34% Dolomite (A calcium CaMg(Co magnesium carbonate) lgloss 44-47;

CaO 32-3571 MgO 17-20% Limestone lg' loss 43-45% CaO 54-57% Zine white ZnO 98% The coefficient of thermal-expansion of the glaze can be varied over a broad range of 3.9 X ]0'0 by controlling the amounts of ZnO, SiO and U 0 in the body without further treatment such as reheating.

A low therrnal-expansion porcelain thus obtained has a smooth, dense, and hard glazed surface and is non-porous. Therefore it has a good appearance as well as durability which makes this porcelain suitable and useful for tableware and industrial applications. Furthermore, it will possess thermal shock-resistance and physical strength because of the great compressive stress in the glaze.

In order to more clearly illustrate the present invention, the following examples are presented:

EXAMPLE 1 A petalite clay composition consisting of:

Total Analysis Petalite 44% (wt.) Ignition loss 6,8Hu-t.)

SiO, 6L8 Plastic clay 45% (wt.) ALO 22.5 Li,0 [.8 Zinc white 6% (wt.) K,0+Na,0 1.0 ZnO 6.0 Silica Sand 5% (wt.) Others 0,!

is finely ground and mixed by ball mill. Ferrous contents and excess water are removed from the mixture. The mixture is then formed to a porcelain body by casting.

Next, a green glaze composition consisting of Total Analysis is finely ground and mixed by ball mill and applied on the surface of the above green or biscuit body. The coated body is then fired at l250-l 300 C.

The characteristics of the product thus obtained are as follows:

Bending strength 650 kg./cm. Linear thermal-expansion coefficient (porcelain body) (mo-600C.) 2.5Xl0 Linear thermal-ex ansion coefficient (glaze) (at 0-600C.) 232x10 Thermal shock resistance (quenching temp. dif.) 360 C. Color white Surface condition mat, dense, and hard EXAMPLE 2 A porcelain product is obtained in the same manner as in Example 1, using a petalite-clay body composition consisting of:

Total Analysis Petalite 65%(WL) Ignition loss llflwt.) SiO, 7l.01(wt.) Plastic clay 227r(wt.) Al o l8.6%(wt.) Li,O 2.bl(wt.) Zinc white 37((WL) K,O+Na,0 l,07(wt.) ZnO J.OZ(W(.) Silica sand lOMwt.) Others O.SZ(wt.)

and a green glaze composition consisting of:

Total Analysis Silica sand l0%(wt.) SiO 69i5%(mol.) M 0; 1 1.3%(moL) Kaolin (calcined) l8%(wt.) CaO 2.7%(molr) Pctalite 557'(wt.) K O+Na,0 0.9%(moL) Li O ZnO 4.9%(m0L) Limestone l0.7%(mol.)

Zinc white The porcelain product so obtained has the following characteristics:

Bending strength 1 600 kg./cm. Linear thermalexpansion coefficient (porcelain 0.8 l0* body) (at 0-600 C.)

Linear thermal expansion coefficient (glaze) 03x10 (at 0-600 C.)

Thermal shock resistance 1 600 C. (quenching temp.

dif.)

Color White Surface condition Mat, dense, and hard EXAMPLE 3 Following the procedure of Example 1 but using a petaliteclay composition consisting of:

characteristics:

Bending strength tEuOOltg/cm. Linear thermal-expansion coefficient (porcelain body) 0.2 l0' (at 0-600 C.)

Linear thermal-expansion coefficient (glaze) 0.0

(at 0-600" C.)

Thermal shock resistance (temp. difference) 620 C. Color White Surface condition Mat. dense and hard EXAMPLE 4 There is obtained a porcelainproduct in the samemanner. in Example 1, using a petalite-clay body composition consisting of:

Total Analysis Petalile 85%(WL) lgnition loss 2. l%(wt.) SiO 72.6%(WL) Plastic clay l47z(wt.) M 0 l9.l%(wt.) Li,0 3.4%(wL) Zinc white IMwt.) K,O+Na,0 l.0%(wt.) ZnO l.07(wt.) Others 0.8%(wt.) and a green glaze composition consisting of:

Total Analysis Silica sand 6%(wt.) SiO 72.8%(moL) M 0 11.0 Kaolin (calcined) lZHWt.) (a0 2.0 K,O+Na,0 0.8 Pctalitc 70%(wt.) Li O 6.l ZnO 7.3 Limestone 3%(wt.) Zinc white 9%(wt.)

The porcelain product soobtained has the following characteristics:

Bending strength Linear thermal-expansion coefficient (porcelain body) (at 0-600 C.) Linear thermal-expansion coefficient (glaze) (at 0-600 C 4) Thermal shock resistance (temp. difference) Color Surface condition White Mat, dense and hard EXAMPLE 5 Following the procedure of Example 1 but using a spodumene-clay composition consisting of:

Spodumene Plastic clay Zinc' white Silica sand and a green glaze composition consisting of:

Silica sand Alumina Petalite Feldspar Talc (calcined) Dolomite Total Analysis 40%(wt.) ignition loss 7.5%(WL) SiO 55.l%(wt.) $0%(wt.) Al,0, 28.2%(wL) Li,0 2.8mm. 5mm. K,o+Na,o 1.4 am. ZnO 5.0% (wt.) 5%(wt.)

Total Analysis 2l%(wtt) SiO 78.7%(rnoL) A1 0 9.9%(moL) 4%(wt.) CaO Loflmol.) 60%(wt.) MgO 3.2%(rnoL) K,O+Na,0 l.5%(mol.) 894m.) Li,0 5.l1(mol.) 4%(WL) 3%(wt.)

The porcelain product so obtained has the following characteristics:

Bending strength Linear thermal-expansion coefficient (porcelain body) (at 0-600 c. Linear thermal-expansion coefficient (glaze) (at 0600 C.)

Thermal shock resistance (temp. difference) Color Surface condition 650kg./cm.

: White Mat, dense and hard EXAMPLE 6 Repeating-the procedure of Example 1 but using a spodumene-c'lay bodycompositionconsisting of:

Spodumene Plastic clay Silica sand Total Analysis and a green glaze composition consisting of:

Silica sand Kaolin'(calcined) Pctalite Limestone Zinc white 60%(wt.) Ignition loss 3.0%(WL) SiO, 67.0%(wt. 20% (Wt.) A; 24.8mm. Li,0 tr/um. 20% (wt) K,0+Na,0 1.0% wt.)

Total Analysis l4%(wt.) Si0 62.9%(mol) M 0 ll.87((mol.) 27/z(wt.) CaO 4.3%(rnoL) K,O+Na,0 0.9% moi.) 33%(wt.) U 0 3.l7z(mol.) ZnO l7.l%(mol.) 6%(wt.) 20%(wt.)

The porcelain product so obtained has the following characteristics:

Bending strength Linear thermal-expansion coefficient (glaze) (at 600 C.)

Thcrmal shock resistance (temp. difference) Color Surface condition Brownish white Mat, dense and hard EXAMPLE 7 There is obtained a porcelain product in the same manner as in Example 1, using a petalite-spodumene-clay body composition consisting of:

The porcelain product so obtained has the following characteristics:

Bending strength 600 kg./cm. Linear thermal-expansion coefficient orcelain body) (at 0-600" C.) 0.3X 10" Linear thermal-ex ansion coefficient (glaze) (at 0-600 C.) 0.0 Thermal shock resistance (temp. difference) 600 C.

Color Surface condition What is claimed is:

1. Process for manufacturing a porcelain body having a low coefficient of thermal-expansion and being provided with a smooth, dense and hard glaze layer, which comprises coating a porcelain body with a glaze preparation, said porcelain body being made up of about 74-995 percent of a composition consisting essentially of at least one mineral selected from the group consisting of petalite and spodumene, in admixture with plastic clay, the total composition containing about 55-74 percent by weight SiO about l8-32 percent by weight M 0, and about 1.5-4.5 percent by weight Li o, said porcelain body, including at least one additive selected from the group consisting of 0.5-6 percent by weight ZnO and 7-26 percent by weight free SiO said glaze preparation consisting essentially of 60 79 mol percent SiO 9-12 mol percent A1 0 O-20 mol percent ZnO, 0-7 mol percent MgO, 2-5 mol percent CaO and 0-7 mol percent U 0, and thereafter firing said coated porcelain body at a temperature of about l250-l350 to vitrify said porcelain and produce in the glaze crystals having a low thermal-expansion coefficient.

2. A process according to claim 1 wherein said porcelain body composition consists essentially of about 40-90 percent petalite and about 10-60 percent by weight of plastic clay.

3. A process according to claim 1 wherein said porcelain body composition consists essentially of about 30-60 percent by weight of spodumene and about 40-70 percent by weight of plastic clay.

4. A process according to claim 1 wherein said plastic clay is kaolin.

5. A method according to claim 1 wherein zinc oxide in the porcelain body is used in an amount of 2-5 percent.

6. Method according to claim 1 wherein the free SiO, in the porcelain body is present in an amount of 7-15 percent. 

2. A process according to claim 1 wherein said porcelain body composition consists essentially of about 40-90 percent petalite and about 10-60 percent by weight of plastic clay.
 3. A process according to claim 1 wherein said porcelain body composition consists essentially of about 30-60 percent by weight of spodumene and about 40-70 percent by weight of plastic clay.
 4. A process according to claim 1 wherein said plastic clay is kaolin.
 5. A method according to claim 1 wherein zinc oxide in the porcelain body is used in an amount of 2-5 percent.
 6. Method according to claim 1 wherein the free SiO2 in the porcelain body is present in an amount of 7-15 percent. 